1. Field of the Invention
The present invention relates to a semiconductor device. More specifically, the present invention relates to the active region where a transistor is formed on a semiconductor substrate.
2. Description of the Related Art
A transistor is a main component of a semiconductor chip and comprises source and drain regions and a gate electrode. As an example, a transistor may be used to access a cell capacitor of a semiconductor memory device such as a DRAM. In this application, if a voltage greater than a threshold voltage is impressed across the gate electrode of the transistor, a path along which current can flow (i.e., a channel) is formed between the source and the drain regions. Accordingly, information can be stored in or read from the cell capacitor along such a channel. On the other hand, the memory device is considered to malfunction if current flows between the source and the drain regions before a threshold voltage is impressed across the gate electrode.
However, conventional semiconductor devices have a limitation that will now be explained with reference to FIG. 1. In manufacturing a conventional semiconductor device, a semiconductor substrate is etched to form a trench having a predetermined depth. A device isolation region 18 is formed by filling the trench with insulation material. An active region 20 is defined by the isolation region 18. Gate electrodes 24 are formed so as to extend over the active region 20 and the device isolation region 18. Impurity ions are implanted into the active region exposed by the gate electrodes 24 to form source and drain regions.
In the case of a conventional semiconductor device, the active region 20 is linear and has a uniform width as measured in the direction along which the gate electrodes 24 run. That is, a central part 20A of the active region located between the gate electrodes 24, ends 20B of the active region, and channel parts 20C of the active region have identical widths a, b and c.
In a conventional semiconductor memory device having an active region as described above, the threshold voltage of the transistor is dictated by the minimum line width Lg of the gate electrode. Accordingly, as semiconductor devices become more highly integrated, the minimum line width Lg of the gate electrodes decreases. That is, the exposed active regions 20A and 20B become larger while the threshold voltage becomes smaller. Highly integrated devices having correspondingly small threshold voltages are prone to producing leakage current that causes the devices to malfunction. The threshold voltage could be increased to solve the problem of leakage current by merely producing a device having a larger minimum line width Lg. However, in this case, the exposed active regions 20A and 20B would have a correspondingly smaller area, whereby the device would offer increased resistance. That is, less current would flow through the source and the drain regions when the transistor is turned on.